Controlling the emission of hazardous organic compounds and acid gases from thermal combustion devices is an increasingly important environmental and regulatory issue. Emission of hazardous organic compounds and acid gases can result from hazardous organic compounds present in the feedstocks and/or fuels combusted in thermal combustion devices. Alternatively, thermal combustion devices may operate in a manner causing the formation of hazardous organic compounds and acid gases as products of incomplete combustion.
Due to environmental problems associated with thermal combustion devices emitting hazardous organic compounds and acid gases, the U.S. Environmental Protection Agency regulates thermal combustion devices under the Resource Conservation and Recovery Act and the Clean Air Act. Examples of regulated thermal combustion devices are, but not limited to, boilers and industrial furnaces which use recyclable hazardous wastes as fuels and feedstocks, and hazardous waste incinerators which are used to destroy hazardous wastes. Operators of regulated thermal combustion devices are required to demonstrate that the device meets a Destruction and Removal Efficiency (DRE) of 99.99% for certain organic compounds fed into the device.
At the present time, measuring the DRE is an expensive and time-consuming process that is not, and generally cannot be, performed on a routine or ongoing basis. Generally, infrequent DRE measurements are performed to insure that a given thermal combustion device is actually operating at or above the required DRE.
There is a need, therefore, to develop a method and apparatus capable of monitoring toxic organic compounds, and their combustion products. As a part of a responsible environmental policy, it is also important for industrial operators to demonstrate to local communities that an emission source, such as a thermal combustion device, is routinely operating in compliance with and well within environmental guidelines.
Recently, there have been promulgated standards for the operation of thermal combustion devices, combusting hazardous waste materials and forming a gaseous effluent, requiring them to demonstrate the effective removal or destruction of organic compounds within the material combusted. As used here, a thermal combustion device is any device which uses controlled flame, infrared, plasma arc or other means to generate high temperature combustion of organic bearing materials and which generate gaseous effluent from the combustion process. Generally speaking, thermal combustion devices must demonstrate a destruction and removal efficiency (DRE) of 99.99 percent for one or more principal organic hazardous constituents (POHCs) in the material to be combusted. The POHCs are chosen based on the degree of difficulty of destruction or their "hard-to-burn" ranking. Examples of the POHCs which can be chosen for a DRE determination are listed in Appendix VIII (Hazardous Constituents) to 40 C.F.R. .sctn.261, which is incorporated herein by reference.
Exposure to high levels of certain organic compounds has been shown to result in a variety of acute and toxic effects in animals. These effects can include damage to liver and kidneys, as well as to the central nervous system and the cardiovascular system. Carcinogenic effects, at least in animals, have also been demonstrated from exposure to certain organic chemicals. Benzene, vinyl chloride, carbon tetrachloride, 1,2-dichloroethane, trichloroethylene, and tetrachloroethylene, are considered to be known or probable carcinogens in humans.
The percent destruction and removal efficiency (DRE) of a thermal combustion device for a given organic compound or POHC is defined by the following equation: EQU DRE=100.times.(W.sub.in -W.sub.out)/W.sub.in
where:
W.sub.in =mass feed rate of an organic compound into the device; and PA1 W.sub.out =the mass emission rate of the organic compound in the gaseous effluent. PA1 W.sub.in =mass feed rate of an organic compound into the device, and PA1 W.sub.out =the mass emission rate of the organic compound in the gaseous effluent. PA1 W.sub.in =mass feed rate of an organic compound into the device, and PA1 W.sub.out =the mass emission rate of the organic compound in the gaseous effluent. PA1 W.sub.in =mass feed rate of an organic compound into the device, and PA1 W.sub.out =the mass emission rate of the organic compound in the gaseous effluent.
Conventionally, the destruction and removal efficiency of a thermal combustion device is determined using a trial burn. In a trial burn, a known organic compound is fed into the thermal combustion device at a known rate with the material normally combusted in the device. This provides the mass feed rate, W.sub.in. In the typical trial burn, the organic compound is fed into the thermal combustion device for at least two hours per test and each test is generally repeated three times.
After destruction, usually by combustion, in the thermal combustion device, the organic compounds present in the gaseous effluent within the stack are collected in a trap for a period of time, generally two hours, to concentrate the sample. A commonly used device is a volatile organic sample train (VOST) sampler equipped with a Tenax.RTM. trap. The trap is then sent to a laboratory where the organic compounds are desorbed and analyzed usually by gas chromatography and/or mass spectrometry techniques, to determine the mass emission rate, W.sub.out. Having directly determined W.sub.in and W.sub.out in this manner, the DRE of the thermal combustion device for the organic compound of interest can then be calculated.
This type of direct DRE determination for a thermal combustion device suffers from several disadvantages. If a thermal combustion device is operating at 99.99 percent DRE, the concentration of an organic compound in the gaseous effluent is too low to permit its concentration to be determined from a single unconcentrated sample. Thus, the gaseous effluent sampled is concentrated to allow the organic compounds present in the effluent to be analyzed. The concentrated sample is then analyzed off-site. This does not allow for real time determination of the DRE of a thermal combustion device to ensure that it is operating within the appropriate environmental guidelines.
The cost of a trial burn is also a disadvantage. Generally, an outside crew is hired to perform the trial burn. Additionally, the cost of the known organic compound or POHC being used in the trial burn is not insignificant.
Another significant disadvantage is the time requirements of a trial burn. First, a typical trial burn requires two to three days to set up and perform the test. Secondly, there is often a significant time delay between when the trial burn is run and when the results are available to the operator.
Due to the disadvantages associated with a trial burn, these tests are typically performed only at intervals of three to five years. Consequently, there is no assurance that operation of the thermal combustion device between test intervals is consistent with the operation of the device during the trial burn or within the appropriate environmental guidelines.
Therefore, one object of the present invention is to provide an inexpensive method of determining the destruction and removal efficiency of a thermal combustion device without the necessity of using large amounts of material containing hazardous organic compounds.
A second object of the invention is to provide an indirect method for assessing the destruction and removal efficiency of a thermal combustion device.
A third object of the invention is to provide a method and apparatus for assessing the destruction and removal efficiency of a thermal combustion device in real time.
Other objects and advantages of the present invention will be apparent from the following specification or would be readily apparent to those skilled in the art to which this invention pertains.